1. Field of the Invention
The present invention relates to an adsorbent state determining apparatus for determining the state of an adsorbent arranged in an exhaust system of an internal combustion engine for adsorbing hydrocarbons and moisture in exhaust gases to purify the exhaust gases.
2. Description of the Prior Art
Some internal combustion engines are provided with an adsorbent for adsorbing hydrocarbons in exhaust gases upon starting. The adsorbent has, for example, zeolite on the surface such that hydrocarbons in exhaust gases introduce into pores of zeolite when they pass through the adsorbent, and are adsorbed by the adsorbent. The adsorbent desorbs the hydrocarbons once adsorbed thereby when the adsorbent is heated by exhaust gases to a predetermined temperature or higher (for example, 100-250xc2x0 C.), permitting the desorbed hydrocarbons to be recirculated to the internal combustion engine through an EGR pipe and the like. While the adsorbent repeats the adsorption and desorption of hydrocarbons as described above, the remaining amount of hydrocarbons not desorbed may gradually increase in the adsorbent, or pores of the adsorbent may be broken during a long-term use. As a result, the adsorbent is deteriorated, causing a gradual degradation in the ability of the adsorbent to adsorb the hydrocarbons. When the internal combustion engine is started with such a deteriorated adsorbent, hydrocarbons not adsorbed by the adsorbent are emitted to the outside. It is therefore necessary to determine the state, more particularly, a deterioration of the adsorbent.
The applicant has already proposed a deterioration determining apparatus for determining a deterioration of an adsorbent as described above, for example, in Japanese Patent Application No. 2000-338375. This deterioration determining apparatus relies on a proportional relationship between the abilities of the adsorbent to adsorb hydrocarbons and moisture to determine a degradation in the abilities of the adsorbent to adsorb hydrocarbons and moisture, i.e., a deterioration of the adsorbent by detecting the humidity in exhaust gases using a humidity sensor after they pass the adsorbent. This adsorbent deterioration determination is executed when conditions for the execution are regarded as established, i.e., when an engine water temperature is within a predetermined range, when an absolute humidity of the internal combustion engine upon starting, detected by the humidity sensor, is within a predetermined range, and the like.
The humidity sensor comprises a sensor element which is made of a porous material having a large number of pores, and detects the humidity of exhaust gases as moisture in the exhaust gases introduces into the pores and is adsorbed therein when it passes through the sensor element. In this way, the humidity is detected with the sensor element exposed to exhaust gases, so that impurities such as water droplets produced by condensation, unburnt fuel components included in the exhaust gases, and the like can stick to the sensor element, in which case the humidity sensor fails to correctly detect the humidity of exhaust gases, resulting in the inability to correctly detect a deterioration of the adsorbent. To solve this problem, the deterioration determining apparatus cleans the sensor element using a heater for a predetermined time period for removing water droplets and the like sticking on the sensor element to recover the detection accuracy of the humidity sensor. Specifically, the heater is operated for the predetermined time period when the intake temperature is lower than a predetermined temperature before the start of the internal combustion engine, or when an idling operation continues for a predetermined time period or longer after the start of the internal combustion engine, on the assumption that the sensor element experiences condensation.
The foregoing deterioration determining apparatus, however, still leaves room for improvement in the following aspects. The deterioration determining apparatus executes a deterioration determination for the adsorbent when the engine water temperature is within a predetermined temperature range and when the absolute humidity upon start of the engine is within a predetermined range, on the assumption that the conditions for execution are established. Thus, impurities such as unburnt fuel components are possibly sticking on the sensor element upon establishment of the conditions for executing the deterioration determination, so that the deterioration determination is executed for the adsorbent with such impurities sticking on the sensor element. As a result, the humidity sensor suffers from a degraded detection accuracy caused by the impurities sticking on the sensor element, and accordingly fails to provide a sufficient accuracy for the deterioration determination.
The present invention has been made to solve the problem mentioned above, and it is an object of the invention to provide an adsorbent state determining apparatus which is capable of accurately determining the state of an adsorbent, including a deterioration, while ensuring elimination of the influence of impurities sticking on a sensor element of a humidity sensor.
To achieve the above object, the present invention provides an adsorbent state determining apparatus for determining a state of an adsorbent disposed in an exhaust system of an internal combustion engine for purifying exhaust gases, where the adsorbent is capable of adsorbing hydrocarbons and moisture within exhaust gases. The adsorbent state determining apparatus is characterized by comprising a humidity sensor disposed near the adsorbent in the exhaust system for detecting a humidity of exhaust gases; cleaning executing means for executing cleaning for removing impurities sticking on a sensor element of the humidity sensor; cleaning completion determining means for determining whether or not the cleaning executing means has completed the cleaning; and adsorbent state determining means for determining the state of the adsorbent in accordance with the humidity of exhaust gases detected by the humidity sensor when the cleaning completion determining means determines that the cleaning is completed.
According to this adsorbent state determining apparatus, the cleaning executing means executes the cleaning for removing impurities sticking on the sensor element, and the adsorbent state determining means determines the state of the adsorbent in accordance with the humidity of exhaust gases detected by the humidity sensor when the cleaning completion determining means determines that the cleaning is completed. Thus, the state determination can be carried out using the humidity of exhaust gases which is detected when no impurities are sticking on the sensor element, thereby making it possible to accurately determine the state of the adsorbent while ensuring elimination of the influence by the impurities.
Preferably, in the adsorbent state determining apparatus, the cleaning executing means comprises a heat cleaning means for heating the sensor element of the humidity sensor to remove impurities.
According to this preferred embodiment of the adsorbent state determining apparatus, since the impurities are removed by the heat cleaning which heats the sensor element, it is possible to burn impurities sticking on the sensor element, for example, water droplets, unburnt fuel components, and the like without fail. Consequently, a high accuracy can be held for the detection of the humidity in exhaust gases by the humidity sensor, so that the state of the adsorbent can be accurately determined.
Preferably, in the adsorbent state determining apparatus, the adsorbent state determining means determines a deterioration of the adsorbent as the state of the adsorbent.
As described above, the abilities of the adsorbent to adsorb hydrocarbons and moisture are in a proportional relationship to each other, so that the humidity detected by the humidity sensor has a high correlation to hydrocarbons actually adsorbed in the adsorbent, i.e., the degree of deterioration of the adsorbent. According to the present invention, therefore, a deterioration of the adsorbent can be accurately determining while ensuring elimination of the influence by impurities.